The invention relates to an osteosynthesis screw having a shaft, provided at least partially with a thread, as well as a tool engagement area provided at least at one end.
Osteosynthesis screws of this kind are known in various embodiments. In this connection, it was attempted to provide an improvement by a special embodiment of the head of the screw, a penetration tip at the end facing away from the head, or by a special configuration of the threaded area. It has also already been attempted to provide a reverse-lock safety for the screws which, however, has been possible only partially in combination with osteosynthesis plates to be fastened and with special additional design expenditure. A rotary locking safety is provided, of course, also in the case of bone marrow nails which are provided with longitudinal ribs or grooves. In the case of such bone marrow nails there is however no possibility for an axial securing action. In the case of locking nails there is the possibility of axial securing.
From U.S. pat. No. 5,026,374 an orthopedic-surgical screw is known which is used in a device, for example, a mono-axial extension device, for external fixation. This known screw has a thread-free central portion which in cross-section is drop-shaped and is provided with an axially extending cutting edge. This central portion does not come into contact with the bone but only with the soft tissue external to the bone. Moreover, the known screw has a cylindrical end portion provided with an outer thread which is provided for screwing into bone material. When this end portion is screwed into a bone, the screw is not secured against rotation.
The present invention has therefore the object to provide an osteosynthesis screw of the aforementioned kind with which in a simple way a rotary locking action as well as a safety against self-acting axial displacement is possible.
According to the invention this is achieved in that the area of the shaft with the thread and/or a thread-free area of the shaft engaging the bone is noncircular, in the manner of an orbiform curve, for example, trilobular or polygonal.
With this feature according to the invention it is achieved that the osteosynthesis screw can be screwed in like a screw, wherein the area of the shaft with the thread by means of thread engagement provides an axial securing action against a self-acting displacement. With the embodiment of the area with the thread and/or a possibly present thread-free area according to the present invention, an optimal rotary locking action is provided also. However, the tight seat of a screw in the drilled hole occurring in metal construction is not employed in this connection because especially in the area of a bone these radial forces cannot act on the screw. By means of the noncircular, or the like, cross-sectional shape of the threaded area or of a thread-free area, a positive-locking connection between the bone material and the displacement in the axial direction as a result of the presence of a thread.
In spite of this, a later loosening of such a screw is possible in a simple manner. The ingrowing bone will yield already upon application of a small torque onto the osteosynthesis screw so that it can be unscrewed without problems if this is necessary. As a result of the noncircular configuration it is only achieved that the osteosynthesis screw cannot self-actingly become loose and then unscrew itself. Accordingly, only minimal forces are required for the position securing action against rotation and against axial displacement.
In order to be able to screw the osteosynthesis screw in a conventional manner easily into a pre-drilled hole, it is provided that the thread on the shaft is self-tapping and/or self-cutting. The osteosynthesis screw acts thus always in a self-shaping way with respect to producing the thread in the drilled hole.
One embodiment provides that the thread and the shaft in this area are cylindrically designed, wherein the thread-free area of the shaft adjoining the area of the shaft with the thread at one or both ends is noncircular, in the fashion of an orbiform curve, for example, trilobular or polygonal, and/or is provided with projections and/or depressions. Accordingly, an osteosynthesis screw can be used in which a conventional thread is employed, wherein then however a thread-free area engaging the bone area is noncircular or the like. In such a case the thread provides the securing action against an axial displacement of the osteosynthesis screw and the thread-free area of the shaft, which is noncircular or the like, effects the rotary locking action by positive-locking engagement with the ingrowing bone. action against rotation and against axial displacement.
In order to be able to screw the osteosynthesis screw in a conventional manner easily into a pre-drilled hole, it is provided that the thread on the shaft is self-tapping and/or self-cutting. The osteosynthesis screw acts thus always in a self-shaping way with respect to producing the thread in the drilled hole.